Minnesota Plant Life. 41 



an habitation. Yet on account of their various structural pecu- 

 liarities botanists assign them to the group of red algae. 



General remarks about algae. The account that has been 

 given is very elementary and the reader must remember that it 

 covers perhaps as many as a thousand varieties, most of which, 

 are species of bright-green algae and diatoms. None of the 

 fresh-water algae has any great economic importance. Some 

 sea-weeds are employed in the manufacture of iodine, others, 

 especially the kelps, as fertilizing material for farms near the 

 sea-shore. The "Irish moss," as it is called, is a red alga and 

 is used for food; when cooked it is a kind of blanc-mange. In 

 China several other kinds of sea-weed are regarded as edible. 

 The Indian fishermen in Alaska use the stem of the giant kelp 

 as siphons and for fishing lines. In Minnesota the algae are 

 sometimes rather noxious than useful. Blue-green algae in 

 decaying masses are known to give to the water a characteristic 

 pig-pen odor which is very offensive. It is at times a difficult 

 problem to prevent them from vitiating aqueducts and reser- 

 voirs, and cattle are reported to have been poisoned by drinking 

 water which contained their rotting remains. It is, therefore, 

 not the water which contains the bright grass-green pond-scums 

 that is so objectionable, though on account of the slimy charac- 

 ter of these plants they are more repugnant to most people than 

 the verdigris-colored water-flower. Cattle should not be allowed 

 to drink from pools in which the algal vegetation is of a blue- 

 green shade, but no injury is likely to result if the scums are 

 bright-green. 



In past time it should be remembered that certain lime- 

 secreting-algae and silica-secreting algae have no doubt done 

 their part in creating the building-stones of the state. Even the 

 quartzites and the granites may be the modified sinter deposits 

 from some hot-water algal vegetation of former ages. In the 

 sea, to-day, countless millions of algae are busy building coral- 

 reefs similar to those produced by the coral polyp, while nearer 

 home, in Lake Michigan, limestone pebbles have been found to 

 be produced by the concretionary growth of lime-secreting 

 algae. I have not yet found any of these algal pebbles in the 

 lakes of Minnesota, but it is probable that they occur. If any 

 one should chance to find calcareous pebbles the size of an egg, 



